1. Field of the Invention
The present invention relates to optical receiving devices and optical receiving methods for receiving an optical signal via an optical fiber and, more particularly, relates to an optical receiving device and an optical receiving method for outputting a received signal or stopping outputting the received signal in accordance with a received optical power of an optical signal.
2. Description of the Background Art
In optical communications, a received optical power satisfying a bit error rate of 10−12 is usually defined as the minimum received optical power, and an optical signal should be received at a received optical power greater than the minimum received optical power. When a received optical power of an optical signal is smaller than the minimum received optical power, a code error occurs frequently, whereby an optical receiving device which receives an optical signal cannot determine the logic states “1” and “0” of a digital signal correctly. Thus, some conventional optical receiving devices have a function of outputting a signal when a received optical power of an optical signal is greater than a predetermined detection level, and stopping outputting the signal when a received optical power of the optical signal is smaller than the predetermined detection level (for example, Japanese Laid-Open Patent Publication No. 10-112689 (patent document 1)).
FIG. 14 is a block diagram showing an exemplary structure of a conventional optical receiving device 901. In FIG. 14, the conventional optical receiving device 901 includes an optical-to-electrical conversion section 902, a received optical power detection section 903, a detection level setting section 904, a comparison section 905, and an output/stop switching section 906. An optical signal is inputted to the optical-to-electrical conversion section 902 via an optical fiber 907. The optical-to-electrical conversion section 902 converts the inputted optical signal to an electrical signal. The electrical signal converted by the optical-to-electrical conversion section 902 is inputted to the received optical power detection section 903 and the output/stop switching section 906. Based on the inputted electrical signal, the received optical power detection section 903 detects a received optical power P of the optical signal. The detection level setting section 904 outputs a predetermined detection level L used for comparison with the received optical power P.
The comparison section 905 compares the received optical power P detected by the received optical power detection section 903 with the detection level L outputted from the detection level setting section 904, and notifies the comparison result to the output/stop switching section 906. In the case where the received optical power P is greater than the detection level L, the output/stop switching section 906 outputs the electrical signal converted by the optical-to-electrical conversion section 902. On the other hand, in the case where the received optical power P is smaller than the detection level L, the output/stop switching section 906 stops outputting the electrical signal converted by the optical-to-electrical conversion section 902. As a result, the conventional optical receiving device 901 can output a signal when the received optical power P of the optical signal is greater than the predetermined detection level L, and stop outputting the signal when the received optical power P of the optical signal is smaller than the predetermined detection level L.
However, the conventional optical receiving device 901 has the following drawbacks. Specifically, when the optical receiving device 901 is connected to the optical fiber 907, the received optical power P of the inputted optical signal fluctuates, such that the received optical power P reaches the detection level L at a particular point, and soon returns to a level below the detection level L, whereby the optical receiving device 901 stops outputting the signal. This is because the received optical power P of the optical signal is unstable until a plug provided on the optical fiber 907 is fully connected to a receptacle or a connector provided on the optical receiving device 901 when the optical receiving device 901 is connected to the optical fiber 907.
The above-described drawbacks of the conventional optical receiving device 901 will be described by using FIGS. 15 and 16. FIG. 15 is an illustration for describing a drawback of the optical receiving device 901 in the case where a continuous signal is received. In FIG. 15, the received optical power P of the optical signal exceeds the detection level L at a time t1, returns to a level below the detection level L at a time t2, and exceeds the detection level L again at a time t3. In this case, the conventional optical receiving device 901 outputs the signal from the time t1 to the time t2, and stops outputting the signal from the time t2 to the time t3. As a result, the signal outputted by the optical receiving device 901 is interrupted from the time t2 to the time t3, and data is lost during that interruption.
FIG. 16 is an illustration for describing a drawback of the optical receiving device 901 in the case where a burst signal is received. In FIG. 16, assume that the optical receiving device 901 receives control signals S1 to S3 as a burst signal prior to receiving a data signal S4. The above control signals S1 to S3 are transmitted from a system such as IEEE1394b-2002. The received optical power P of the control signal S1, which has been received first, is equal to or greater than the detection level L. Thus, the optical receiving device 901 outputs the control signal S1 as an output signal. However, the optical receiving device 901 stops outputting the control signal S2 because the received optical power P of the control signal S2 is smaller than the detection level L. As a result, the device which receives a signal outputted from the optical receiving device 901 becomes unable to receive the control signal S2, thereby failing to establish communications between the devices.
With some protocols once the establishment of communications between the devices has failed the communications between the devices would stop and never resume. Therefore, in the case where the establishment of communications between the devices has failed, the conventional optical receiving device 901 is required to reset the device or reconnect the optical fiber 907 in order to recover the communications.